38 
TERTIAliT VEETEBRATA OF THE FAYtM. 
lobe is nearly vertical and does not extend so liigli np as the upper end of the 
olecranon ])roccss. Of the two horizontal surfaces the inner (i.) is much the larger 
and is oval in outline, its antero-posterior diameter being slightly greater than the 
transverse ; it is concave in all directions. The outer (o.) is the smaller and is 
semicircular in outline, the diameter of the semicircle looking forwards and inwards 
and marking the line of union with the radius, with which the bone unites in a 
narrow vertical surkice (r.s.). Immediately beneath the transverse articulation is 
Text-fig. 18. 
Left ulna of Arsinoitlierium zitteli ; A, from front ; B, from outer side ; C, proximal end (with radius) ; 
D, distal end (with radius). 
cot., surface for cuneiform ; e.p., external prominence ; g., groove above distal articular end ; i., inner surface 
for humerus; L, surface for lunar; o., outer surface for humerus; o/., olecranon process; »•., radius; 
r.s., proximal surface for radius ; r.s.', distal surface for radius ; sc., surface for scaphoid ; u., ulna. 
I nat. size. 
a deep fossa which receives the head of the radius. The shaft of the bone is 
triangular in section, the posterior angle being continuous with the hinder border 
of the oh'cranon process. The shaft is also curved somewhat backwards and 
inwards. Towards the distal end, which is considerably enlarged, the inner border 
